FIG. 7 is a cross sectional view showing a typical prior art of this kind of flexible coupling 100 by cutting along a plane orthogonal to an axis thereof, and FIG. 8 is a cross sectional view along a line VIII-VIII in FIG. 7. In other words, the flexible coupling 100 shown in FIGS. 7 and 8 is structured such that a plurality of and an equal number of drive side connection elements 101 and driven side connection elements 102 are alternately arranged in a circumferential direction, the drive side connection element 101 and the driven side connection element 102 which are adjacent in the circumferential direction are connected by first and second connection bands 105, 106 which are wound in a loop shape around a drive side bobbin 103 and a driven side bobbin 104 respectively attached to outer peripheries thereof and are formed by winding a cord such as a polyester or the like having a suitable tensional elasticity in a multilayer shape, and the bobbins 103, 104 and the connection bands 105, 106 are integrally buried in an annular elastic body 107 made of rubber or the like.
Each of the driven side and driven side bobbins 103, 104 is constituted by a steel sleeve 108 and a pair of steel collars 109 which are pressure inserted and fixed to an outer periphery of the sleeve 108 at a predetermined interval in an axial direction. Each of the collars 109, 109 is press molded in an approximately C cross sectional shape having an annular collar portion in both sides in an axial direction.
The first connection band 105 is wound around a portion between the collars 109, 109 in the outer peripheral surface of the sleeve 108 of the drive side bobbin 103, and around a portion between the collars 109, 109 in the outer peripheral surface of the sleeve 108 of the driven side bobbin 104, and the second connection bands 106, 106 adjacent thereto in the circumferential direction are wound around the collars 109, 109 of the drive side bobbin 103 and around the collars 109, 109 of the driven side bobbin 104, respectively. In other words, each of the drive side connection elements 101 and each of the driven side connection elements 102 which are alternately arranged at a uniform interval in the circumferential direction are connected alternately in the circumferential direction by a bundle of first connection band 105 and two bundles of second connection bands 106, 106.
The flexible coupling 100 is structured such that the drive side connection element 101 in the inner periphery of the drive side bobbin 103 is mounted to a yoke in an axial end of a drive side rotation shaft via bolts and nuts (not shown) arranged at a uniform interval in a circumferential direction, and the driven side connection element 102 in the inner periphery of the driven side bobbin 104 is mounted to a yoke in an axial end of a driven side rotation shaft via bolts and nuts (not shown) arranged at a uniform interval in a circumferential direction. Accordingly, it is possible to transmit a rotation torque of the drive side rotation shaft to the driven side rotation shaft, it is possible to allow a rotation transmission in a connection state in which axial directions of the drive side rotation shaft and the driven side rotation shaft are different (a pinch state), and an axial relative displacement of both the rotation shafts on the basis of a deforming characteristic of the first and second connection bands 105, 106 and the annular elastic body 107, and it is possible to absorb a vibration transmitting between both the rotation shafts.
In this kind of flexible coupling 100, the first and second connection bands 105, 106 extend in parallel in an inner peripheral side and an outer peripheral side in the annular elastic body 107 between the bobbins 103, 104. Accordingly, in the case that a relative torsional displacement in a circumferential direction is generated between the drive side bobbin 103 and the driven side bobbin 104 which are adjacent in the circumferential direction, on the basis of an input of the transmission torque, the first connection band 105 or the second connection band 106 are enlarged more largely in portions 105a, 106a extending in an outer peripheral side (hereinafter referred to as an outer peripheral side portion) in comparison with portions 105b, 106b extending in an inner peripheral side (hereinafter referred to as an inner peripheral side portion) within the annular elastic body 107, whereby an elastic force difference is generated.
However, since the first and second connection bands 105, 106 constituted by the polyester cord or the like is hard to slip with respect to the steel bobbins 103, 104 (the sleeve 108 and the collar 109), the elastic force difference between the inner peripheral side portions 105b, 106b, and the outer peripheral side portions 105a, 106a is hard to be cancelled. Accordingly, the tensional force difference is increased in accordance with an increase of the torsional displacement between the drive side bobbin 103 and the driven side bobbin 104. Further, since the torsional torque is applied to the outer peripheral side portions 105a, 106a in the first connection band 105 or the second connection band 106 in a biased manner, under this state, a tensile stress and a strain of the outer peripheral side portions 105a, 106a are increased, whereby a torsion angle is further increased, so that there is a risk that the first and second connection bands 105 and 106 are broken early by fatigue.
Further, in this kind of flexible coupling 100, if a suitable initial slack is applied to the first and second connection bands 105, 106, a torsional rigidity can be maintained small until the slack of the first connection band 105 or the second connection band 106 is cancelled between the drive side bobbin 103 and the driven side bobbin 104 at a time of inputting the torque. Accordingly, it is possible to achieve an excellent vibration absorbing effect, and it is possible to achieve a two-stage characteristic that the torsional rigidity is increased at a time when the first connection band 105 or the second connection band 106 is enlarged linearly, and a great torque transmission force is generated. However, if the slip between the first and second connection bands 105, 106, and the drive side bobbin 103 and the driven side bobbin 104 is hard to be generated, the initial slack of the outer peripheral side portions 105a, 106a and the inner peripheral side portions 105b, 106b in the first and second connection bands 105, 106 is uneven, and it is hard to secure a stable two-stage characteristic.
Further, the drive side bobbin 103 and the driven side bobbin 104 are manufactured by pressure inserting the collar 109 obtained by press molding a steel plate into the outer peripheral surface of the sleeve 108 which is formed by drawing a steel pipe and is chamfered in inner peripheral portions of both end surfaces in accordance with a cutting process. The chamfer of the sleeve 108 is formed for the purpose of making it easy to pressure insert the sleeve 108 to the outer periphery of the drive side connection element 101 or the driven side connection element 102. However, the bobbins 103, 104 manufactured in the manner mentioned above require a drawing process of the steel pipe and a chamfering process by cutting the inner peripheral portions of both end surfaces, at a time of manufacturing the sleeve 108, and a working process for manufacturing the collar 109 having the collar portions in both ends in the axial direction is troublesome. Accordingly, there is a problem that a manufacturing cost is high.
The present invention is made by taking the problems mentioned above into consideration, and a technical object of the present invention is to prevent a connection band from being broken early so as to improve a fatigue resistance, secure a two-stage characteristic of a vibration absorbing performance at an initial time of inputting a torque, and a shock absorbing performance and a torque transmission force, and provide the structure at a low cost.